1. Field of the Invention
The present invention relates to a phase modulator of a radio frequency (RF) transmitter and, more particularly, to a high speed phase modulator employing a polar modulation scheme for multi-band processing and a method of the same.
2. Description of Related Art
A superheterodyne or a polar modulation scheme may be utilized as a transmission method in a system for transmitting/receiving high speed wireless data, such as, for example, a mobile phone, a digital multimedia broadcasting (DMB) phone and a personal digital assistant (PDA).
FIG. 1 is a diagram illustrating a transmitter 100 employing a superheterodyne scheme according to a conventional art. Referring to FIG. 1, the transmitter 100 includes an intermediate frequency processing unit 110, a carrier processing unit 120, a surface acoustic wave (SAW) filter 130 and a power amplifier 140. In this system, the phase-modulated I and Q signals in the baseband are up-converted to an intermediate frequency band in the intermediate frequency processing unit 110. Then, the output of the intermediate frequency processing unit 110 is up-converted to a frequency band of the carrier signal in the carrier filter 120. When the baseband signal is being processed in the intermediate frequency processing unit 110 and the carrier processing unit 120, an image tone, analogous to noise, may be generated. When this occurs, the generated image tone is removed in the SAW filter 130. The signal output from the SAW filter 130 is amplified by the power amplifier 140 to an appropriate power level.
However, the conventional transmitter 100 employing a superheterodyne scheme as described above requires the SAW filter 130 in order to remove the image tone. Therefore, because the power amplifier 140 has to cover the resulting changes in linearity for a modulation scheme such as, for example, quadrature phase shift keying (QPSK) and binary phase shift keying (BPSK), the transmitter 100 is not suitable for multi-band processing.
In contrast, a polar modulation scheme is suitable for multi-band processing in an ubiquitous system that can up-convert a baseband signal in a Code Division Multiple Access (CDMA) system, a global positioning system (GPS), a personal communication system (PCS), an International Mobile Telecommunication (IMT) 2000 system, Wireless Broadband Internet (WiBro) system, a wireless local area network (WLAN), an Ultra Wideband (UWB) system and a WiMax system.
FIG. 2 is a diagram illustrating a transmitter 200 employing a polar modulation scheme according to a conventional art. Referring to FIG. 2, the transmitter 200 includes a phase-locked loop (PLL) 210, a voltage-controlled oscillator (VCO) 220, a power amplifier 230, a modulation unit 240 and a controller 250. In the polar modulation scheme described above, the operation of the PLL 210 and the modulation unit 240 is in accordance with baseband signals I and Q. The PLL 210 and the modulating unit 240 control the phase of the passband signal that is output from the VCO 220. Also, as the phase is shifted, the power level of the signal is appropriately amplified before and after the phase shift by the controller 250.
However, the conventional transmitter 200 employing a polar modulation scheme as described above shifts the phase of the VCO 220 using the PLL 210. Accordingly, a loop filter (not shown) that is provided in the PLL 210 needs a fast response time in order to provide a fast phase shift. However, high speed phase modulation is difficult to achieve because of loop bandwidth limitations. Also, in the polar modulation scheme that directly controls the VCO 220 in order to shift the phase, a momentary frequency change is unavoidable at the instant of the phase shift. In this case, it may not be possible to design a power amplifier that has a sufficiently wide bandwidth corresponding to the frequency change.